defining a variadic coordinate (tuple) type in C++17?

Question

I wanted to define a variadic tuple type to represent coordinates. For example, for some magic type:

template <unsigned int N>
struct CoordT {
  typedef std::tuple<_some_magic_> coord_type;
};

I'd like to have CoordT<3>::coord_type to be the 3-dimensional coordinate type:

std::tuple<double, double, double>

.

But I don't know how to use template programming to generate N repeated double s.

Can anyone please help explain how to write it?

Answer 1

Use std::make_integer_sequence to generate a pack of the appropriate length, then map the elements to doubles:

template <size_t n>
struct TupleOfDoubles {
    template <size_t... i>
    static auto foo(std::index_sequence<i...>) {
        return std::make_tuple(double(i)...);
    }
    using type = decltype(foo(std::make_index_sequence<n>{}));
};

http://coliru.stacked-crooked.com/a/7950876813128c55

Answer 2

If you don't literally need a std::tuple , but just need something which acts like a tuple, use std::array :

template <unsigned int N>
struct CoordT {
  typedef std::array<double, N> coord_type;
};

std::array has overloads for std::get<I> , std::tuple_size , and std::tuple_element . Most library and language facilities which accept a tuple-like element will support std::array , such as std::apply and structured bindings .

Answer 3

Is too late to play?

If for you is acceptable to declare (no definition is needed) a variadic template function as follows

template <std::size_t ... Is>
constexpr auto toIndexSeq (std::index_sequence<Is...> a)
   -> decltype(a);

and that the coord_type is defined in a CoordT specialization, you can write it as follows

template <std::size_t N,
          typename = decltype(toIndexSeq(std::make_index_sequence<N>{}))>
struct CoordT;

template <std::size_t N, std::size_t ... Is>
struct CoordT<N, std::index_sequence<Is...>>
 { using coord_type = std::tuple<decltype((void)Is, 0.0)...>; };

The following is a full C++14 compiling example

#include <tuple> 
#include <type_traits> 

template <std::size_t ... Is>
constexpr auto toIndexSeq (std::index_sequence<Is...> a)
   -> decltype(a);

template <std::size_t N,
          typename = decltype(toIndexSeq(std::make_index_sequence<N>{}))>
struct CoordT;

template <std::size_t N, std::size_t ... Is>
struct CoordT<N, std::index_sequence<Is...>>
 { using coord_type = std::tuple<decltype((void)Is, 0.0)...>; };


int main()
 {
   using t0 = std::tuple<double, double, double, double>;
   using t1 = typename CoordT<4u>::coord_type;

   static_assert( std::is_same<t0, t1>::value, "!" );
 }

Answer 4

A very concise way to do this is by using std::tuple_cat and std::array :

template <unsigned int N>
struct CoordT {
  using coord_type = decltype(std::tuple_cat(std::array<double, N>{}));
};

std::tuple_cat is allowed to support tuple-like types such as std::array , but not guaranteed. However, every implementation I checked supports this .